The present invention relates to a control system for inducing crankshaft rotation and imposing crankshaft position in a hybrid electric vehicle.
A hybrid electric vehicle may be powered alternatively or simultaneously by an internal combustion engine and an electric motor to maximize fuel economy. The electric motor may be part of an electric machine, referred to herein as a motor-generator, which may replace the conventional starter motor and alternator. To move the vehicle from a stopped position, the motor-generator draws electrical energy from a battery pack to turn the engine crankshaft. As vehicle speed increases, fuel and spark are delivered to initiate engine operation. At a certain vehicle speed range, the motor-generator may operate as a generator driven by the engine crankshaft to recharge the battery pack and to supply electrical power to auxiliary vehicle devices such as fans, radios, etc.
When the vehicle is coasting or braking, fuel flow to the engine may be stopped to improve fuel economy. During fuel-off deceleration downshifts, the motor-generator may operate as a motor to synchronize engine and transmission speeds by increasing engine speed to facilitate a downshift. When the engine is off, the auxiliary vehicle devices are powered by the battery pack in cooperation with a DC-DC converter.
If the vehicle is stalled for a period of time, the engine oil pressure gradually decreases in the oil feed galleys to the crankshaft and connecting rod bearings, which may lead to a degradation of the lubricant film and a pure boundary lubrication condition. Due to such an increased friction condition, re-initiating engine start up may require higher torque input from the electric motor-generator to crank the crankshaft.
Another mechanism that can affect the torque required to re-initiate engine start up is the crankshaft angular location during an engine stall. It is favorable to have the crankshaft rest with the intake valve open at the cylinder which is in its intake stroke. If the intake valve closes, the relatively cool inducted air is expanded by the hot cylinder walls, raising the cylinder pressure, hence increasing the required torque to rotate the crankshaft.
As the engine is cranked, the driver may feel compression vibration from the engine. The smoothness of the engine cranking is a function of the rotational position of the crankshaft upon engine start up since the torque required to rotate the crankshaft undulates with the in-cylinder pressures.
The present invention provides a control method for inducing crankshaft rotation in a hybrid electric vehicle. The control method provides the capability of operating an electric motor-generator to affect at what rotational orientation the crankshaft will stop, as the crankshaft speed slows to zero. Further the control method operates the motor-generator to rotate the crankshaft forward or backwards to a more advantageous rotational location once the crankshaft speed is zero, prior to restarting the engine, to minimize the vibration felt by the driver. The control method determines the crankshaft rotational location using a crankshaft locational sensing means which may operate in conjunction with the ignition system.
The control method also provides the capability of powering the electric motor-generator to rotate the crankshaft in one or both rotational directions to redistribute the lubricant film on the crankshaft bearings. This minimizes the need for greater motor torque input to restart the engine as the friction level is maintained and not appreciably increased. The control method measures the time the engine is stalled as a function of the engine coolant temperature and oil pressure to determine when power pulsations to the electric motor are required to xe2x80x9crockxe2x80x9d the crankshaft to re-establish the lubricant film.